Implement GitHub App Manifest Flow

[sodre]

The implementation of the GitHub App Manifest Flow requires three end-points:

• /gh/setup: Called by the application administrator. Presents a UI that lets the Application Administrator choose which Github Organization/User to install the App.
• /gh/setup-callback: Called by GitHub after Application is installed.
• /gh/webhook: Called by GitHub when subscribed events occur.

The /gh/setup endpoint will generate and save a short-lived unguessable random string called state to be received back/confirmed at /gh/setup-callback.

The /gh/setup-callback will exchange its code for an id, client-id, client-secret, webhook-secret and pem. These should be stored in permanent storage encrypted with a key visible only to the App.